How to merge two files in php while replacing the contents? -

i wanna know how merge 2 files in php such 1 final file generated , have both changes,

example :

style.css

#mydiv{    background:white;    color:black; }

and

style2.css

#mydiv{    border:1px solid black; }

and

final.css

#mydiv{    background:white;    color:black;    border:1px solid black; }

so want final.css in output after merging style.css , style2.css,,

note : css file not point, example,, want kind of file merging files both versions.

another example :

file1.js

var x=90; var y=50; var z=x+y;

file2.js

var z=x+y; var yx=500;

and

output.js

var x=90; var y=50; var z=x+y; var yx=500;

thanks!

Swift